1. Field of the Invention
The invention relates to semiconductor lasers, and more specifically to a graded-index, separate confinement heterostructure (GRINSCH) laser.
2. Related Art
Periodically stratified structures are known to have applications as bandpass filters and reflectors in optics, and in practice, they are relatively simple to make and easy to analyze. (See Born and Wolf, Principles of Optics, Pergamon Press, N.Y. (1959) and van der Ziel and Ilegems, Applied Optics, 14(11):2627 (1975).) In a typical semiconductor laser, however, light propagates in the direction parallel to the epitaxial layers. For example, FIG. 1 shows a conventional semiconductor laser 100 having a continuous active channel region 102. As a result, photonic devices such as a distributed feedback laser, which could have been designed to function based on a normal incidence of light on a periodically layered structure, have not been studied extensively.
Recently, however, advances in areas of semiconductor in situ processing (see Harriot et al., J. Vac. Sci. Technol. B, 8(6) (1990)), and in reactive ion etching (see Giapis et al., Appl. Phys. Lett. 57(10):983 (1990), Scherer et al., J. Vac. Sci. Technol. B, 8(1):28 (1990) and Scherer and Craighead, J. Appl. Phys. 49:1284 (1986)), have made possible the fabrication of microstructures with a surface area as small as 100.ANG..times.100 .ANG. (see Rondall et aI., J. Vac. Sci. Technol. B., 8(6) (1990)). Crystal regrowth techniques, on the other hand, have matured to the point that high quality, lattice matched semi-insulating material can be grown routinely. These two major factors, plus the availability of reliable, low cost, and high performance flip-chip solder reflow packaging techniques used in multichip modules (see K. L. Tai, Proceedings of Multichip Module Workshop, IEEE Computer Society, Santa Cruz, Calif. (1991)), demand a re-examination of the role of periodically stratified structures, for photonic device applications.
Given the many advantages a low threshold current offers in a semiconductor laser, there has been a fervent search for the lowest threshold semiconductor laser. The efforts in crystal growth, especially by MBE (molecular beam epitaxy), have resulted in the successful demonstration of sub-100 A/cm.sup.2 (amps per square centimeter) threshold current density SQW (single quantum well) GRINSCH lasers materials. (See Chen et al., Appl. Phys. Lett. 51:2094 (1987) and Electron. Lett. 1334 (1987) and Chand et al., Appl. Phys. Lett. 58:1704 (1991).) Meanwhile, the efforts in mass-etching, crystal regrowth, and mirror-coating lave produced sub-mA (milli-amps) threshold current lasers using SQW GRINSCH structures. (See Derry et al., Appl. Phys. Lett. 50:1773 (1987).)
Despite their significant success, SQW GRINSCH lasers have left much room for improvement. First, the use of a full-cavity length GRIN (graded refractive index) waveguide is overkill for achieving stable, low loss optical oscillation. Second, the reduction of threshold current, which has been mainly due to the reduction of the active volume in both x and y direction (see FIG. 1), is incomplete. Although the use of MBH has enabled the reduction of the active volume of a laser in the x direction, to the quantum limit of several monolayers, and the techniques of mesa-etching and crystal regrowth have reduced the active volume in the y direction to less than 1 .mu.m, the active volume in the z direction along the laser cavity has stayed at the current level of more than 250 .mu.m. What is desired is a semiconductor laser that has minimized threshold current and a reduction of the active volume in the z direction along the laser cavity.